The present invention relates generally to servo systems in disc drives. In particular, the present invention relates to compensation for errors in servo systems.
Disc drives read and write information along concentric tracks formed on discs. To locate a particular track on a disc, disc drives typically use embedded servo fields on the disc. These embedded fields are utilized by a servo subsystem to position a head over a particular track. The servo fields are written onto the disc when the disc drive is manufactured and are thereafter simply read by the disc drive to determine position.
Ideally, a head following the center of a track moves along a perfectly circular path around the disc. However, two types of errors prevent heads from following this ideal path. The first type of error is a written-in error that arises during the creation of the servo fields. Written-in errors occur because the write head used to produce the servo fields does not always follow a perfectly circular path due to unpredictable pressure effects on the write head from the aerodynamics of its flight over the disc, and from vibrations in the gimbal used to support the head. Because of these written-in errors, a head that perfectly tracks the path followed by the servo write head will not follow a circular path.
The second type of error that prevents circular paths is known as a track following error. Track following errors arise as a head attempts to follow the path defined by the servo fields. The track following errors can be caused by the same aerodynamic and vibrational effects that create written-in errors. In addition, track following errors can arise because the servo system is unable to respond fast enough to high frequency changes in the path defined by the servo fields.
Written-in errors are often referred to as repeatable run-out (RRO) errors because they cause the same errors each time the head passes along a track. In general, the position of each servo field has a RRO error. During disc drive manufacture, a correction or compensation table, which includes a series of compensation values for RRO errors in positions of embedded servo fields, is calculated and stored. During subsequent normal operation of the disc drive by the user, the correction or compensation table is used by the servo control loop to improve the alignment of the head over a selected user track.
The compensation values for RRO errors are typically determined during disc drive manufacture through known Zero Acceleration Path (ZAP) compensation schemes. In general, these schemes carry out multiple iterations of RRO measurements for each track. Each iteration involves the averaging of position error signal (PES) values for servo fields of the respective track over multiple disc revolutions to determine RRO values, which are then compared with a preset RRO threshold. Determining RRO values for comparison with a preset RRO threshold by utilizing such an off-line process, which involves collecting PES values over multiple disc revolutions to determine RRO values, is time consuming and a barrier to economical, rapid mass production of disc drives. In this context, an off-line process is one in which the RRO values are not determined continuously or in xe2x80x9creal-time,xe2x80x9d but instead are determined only after collecting PES values over multiple disc revolutions.
Embodiments of the present invention provide solutions to these and other problems, and offer other advantages over the prior art.
The present embodiments relate to a repeatable runout (RRO) identification scheme that is capable of measuring a RRO value of each servo field of a track and comparing the respective measured RRO value with a preset RRO threshold in real-time, thereby addressing the above-mentioned problems.
A RRO identification device for use with a disc drive having a rotatable disc is provided. The RRO identification device determines RRO values for servo fields of a first track of the disc as a function of received PES values for the servo fields independent of averaging the PES values for each servo field. The RRO identification device compares, in real-time, each determined RRO value for each servo field with a threshold repeatable runout value.
Other features and benefits that characterize embodiments of the present invention will be apparent upon reading the following detailed description and review of the associated drawings.